Past attempts to control plant microbial diseases have included the use of chemicals. However, many chemicals that have been in long-time use are now ineffective due to the increasing number of chemical-resistant strains of plant pathogens. Further, many such chemicals are recognized to be potentially hazardous to non-target organisms, particularly humans, and to the environment.
Biological control of plant pathogens, is an alternative to chemical control. It has been recognized that crops grown in some soils naturally are resistant to certain fungal pathogens. Furthermore, soils that are conducive to the development of fungal diseases can be rendered suppressive, or resistant, by the addition of small quantities of soil from a suppressive field. Scher et al., Phytopathology 70:421 (1980). Conversely, suppressive soils can be made conducive to fungal diseases by autoclaving or chemical fumigation, indicating that the factors responsible for disease control are biological. Root colonizing bacteria have been shown to be responsible for this phenomenon, which is known as biological disease control. Baker et al., Biological control of plant pathogens, Freeman Press, San Francisco (1974).
In many cases, the most efficient strains of biological disease controlling bacteria are fluorescent Pseudomonads. Weller et al., Phytopathology, 73: 463-469 (1983). Many biological control Pseudomonas strains produce metabolites, such as antibiotics and sideophores, that inhibit the growth of fungal pathogens. Howell et al., Phytopathology 69: 480-482 (1979); Howell et al., Phytopathology 70: 712-715 (1980).
An important factor in biological control is the ability of an organism to compete in a given environment. Thus, it is desirable to obtain novel biocontrol agents which effectively control (e.g., retard, restraint, kill, lyse) the growth of plant pathogens, particularly fungi, and which are able to aggressively compete with indigenous bacteria and other microflora that can exist on the surfaces or the rhizosphere of the plant.